Presently, optical brighteners are added to make paper appear whiter, instead of a yellowish white. Optical brighteners are provided with fluorescent particles that emit light in the visible wavelength band when excited by ultraviolet light, so that when ultraviolet light interacts with paper that has optical brighteners in it, the optical brighteners will absorb the ultraviolet light and re-emit light at longer wavelengths in the visible wavelength band. The power available in the ultraviolet light, i.e. its intensity, and its wavelength will determine the amount of excitation of the fluorescent particles, and hence the light re-emission in the visible wavelength range by the optical brighteners.
In this description, the term re-emission may be understood as comprising reflectance, fluorescence and phosphorescence. A skilled person will understand that it is actually the combination of re-emitted light sorts, e.g. reflectance, fluorescence and phosphorescence, that contributes to the perceived color of a hard copy.
Colors of certain samples can be verified by color measurement systems. Typically, these systems have one or more light sources for emitting light onto the sample. In one example two different light sources are used, wherein the difference in fluorescent re-emission between them is measured.
In another exemplary measurement instrument a single ultraviolet cut-off filter is used. The filter typically has little or no ultraviolet transmission and is inserted between the light source and the sample of the color measurement instrument. The degree of fluorescence of the sample can be assessed by measuring the re-emission of the sample with and without the filter. In a further example two filters are used that divide the ultra violet band into two ultraviolet bands so that the fluorescence can be measured with more accuracy.
In methods that specifically relate to printing hard copies, the printed hard copy is viewed under one illumination condition, for example an illumination condition that does not include ultraviolet light. If the sample were viewed under a different illumination condition, one that has an ultraviolet component, then the sample could appear different in comparison to the first illumination condition.
It is therefore advantageous to be able to assess the colors and/or re-emission properties of a printed hard copy under different illumination conditions.